Ignition timing of an internal combustion engine may be varied in accordance with engine operating conditions so as to provide optimum engine operation. As an example, for increased engine efficiency and reduced fuel consumption, spark timing for each cylinder may be positioned at minimum spark advance for best torque (MBT). Ignition timing may then be retarded from the optimum timing in response to abnormal combustion events, such as due to knocking. One example approach is shown by Haraldson et al in WO 2011023852. Therein, cylinder-to-cylinder knock control is performed by controlling detonation in each cylinder via independent adjustments to the spark timing of each cylinder.
However, the inventors herein have recognized potential issues with such an approach. While cylinder-to-cylinder knock control allows knock to be addressed more reliably, the exhaust temperature of each cylinder can vary drastically with spark retard (or advance) from other cylinders in the respective bank, as well as between banks (such as in a V-engine). When operating near component temperature limits, such as temperature limits of an exhaust catalyst, an exhaust turbine inlet, one or more exhaust valves, etc., if one or more cylinders has spark retarded from a base value of spark, the exhaust temperatures in those cylinders may exceed design limits. As such, this can reduce the engine's life and increase the need for component replacement. Component temperatures may be controlled by increasing/decreasing fuel delivery across all cylinders of a whole bank (e.g., in an I-engine) or both banks (e.g., in a V-engine). For example, fueling of all cylinders of the engine may be adjusted based on an inferred global engine temperature model with global modifiers for spark and lambse to further predict the change in exhaust temperature for each cylinder. However, this may lead to excess fuel wastage and overall reduced fuel economy and engine performance.
In one example, some of the above issues may be addressed by a method for an engine that enables individual cylinder knock control while maintaining component temperatures within limits with reduced fuel wastage. One example method comprises: maintaining engine exhaust temperature within a threshold via each of spark and fueling adjustments, the spark adjustment based on adaptive knock control values for each of a plurality of engine cylinders, the fueling adjustment based on the spark adjustment.
For example, adaptive knock values may be learned for each cylinder individually, over multiple drive cycles, based on knock occurrences in each cylinder. Based on each cylinder's adaptive knock value, each cylinder may be operated with a different amount of spark retard from MBT. For example, cylinders with a higher propensity for knock may have higher knock adaptive values and may be operated with spark timing retarded further from MBT, while cylinders with a lower propensity for knock may have lower knock adaptive values and may be operated with spark timing less retarded from MBT (e.g., with no spark retard, with spark at MBT, or with spark advanced from MBT). Cylinder fueling may then be adjusted bank to bank based on the learned spark adjustments to enable bank-specific exhaust temperature control. Specifically, for each bank, a cylinder having the largest amount of spark retard may be identified. Fueling of all cylinders of that bank may then be adjusted based on the largest amount of spark retard so as to maintain an exhaust temperature of the given bank below a threshold. As an example, all cylinders of the bank may be enriched based on the largest amount of spark retard. Likewise, spark of cylinders of the other engine bank (such as in a V-engine) may be adjusted based on respective adaptive values while fueling is adjusted based on the cylinder with the largest amount of spark retard. Alternatively, the cylinder with the largest amount of spark retard may be fueled based on the largest amount of spark retard to enable exhaust temperatures to be controlled while fueling of the remaining cylinders of another given bank may be adjusted based on the determined fueling so as to maintain an exhaust air-fuel ratio of the given bank at or around stoichiometry, or any base commanded air-fuel ratio (such as Wide Open Pedal performance—Leanest for Best Torque (LBT)).
In this way, cylinder-to-cylinder knock control can be achieved while compensating for differences in exhaust heat generation due to differential spark adjustment of cylinders. By adjusting cylinder fueling based on the spark modifier of the worst case cylinder, a more accurate prediction of exhaust temperature rise due to spark adjustment can be provided. By fueling all cylinders of an engine bank based on the spark modifier of the cylinder with the largest amount of spark retard (the worst case cylinder), it may better ensured that any cylinder or bank does not exceed temperature limits of engine components. Thus, exhaust temperature control is improved. By calculating the amount of fuel to be added to each engine bank based on a fuel modifier adjusted for exhaust temperature, temperature and knock control can be achieved without wasting fuel. As such, this improves overall engine fuel economy and engine performance.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.